1. Field of the Invention
This invention relates to a semiconductor laser and a manufacturing process therefor.
2. Description of the Related Art
A distributed feedback type semiconductor laser (DFB laser) or distribution Bragg reflection type semiconductor laser (DBR laser) oscillates in a single longitudinal mode and is widely used as a light source for a middle- or long-distance optical communication. In terms of a DFB laser or DBR laser, electron beam lithography has been extensively applied to forming a diffraction grating pattern in order to form a uniform pattern over a whole area with its period being closely controlled.
Japanese Patent Application Nos. 2000-138413 and 1999-354887 have disclosed a technique in terms of a drawing end in such a diffraction grating. According to these references, an average height of a concave bottom after etching is generally different between areas with and without a diffraction grating, in a DFB laser whereby a diffraction grating is formed by electron beam lithography. Thus, when, for example, an active layer is grown on a substrate having this diffraction grating, a step is sometimes formed particularly near a boundary between the areas with and without the diffraction grating, leading to deterioration in crystal quality in a growth layer.
For dealing with the phenomenon, Japanese Patent Application No. 2000-138413 has disclosed a technique that a diffraction grating pattern is formed by electron beam lithography while an area without diffraction grating is exposed to Deep UV. Thus, an area covered by a resist can be reduced to minimize a step between the areas with and without a diffraction grating. Japanese Patent Application No. 1999-354887 has disclosed a technique for dealing with the phenomenon that a step formed after etching can be controlled by modifying an exposure pattern.
As means for reducing a capacity due to a current block layer when constructing a mesa-electrode type structure in a buried waveguide type semiconductor laser to permit high-speed response, there have been disclosed a mesa-electrode structure in which grooves are formed in both ends of an active layer, by K. Kamite, H. Sudo, M. Yano, H. Ishikawa, H. Imai, “Ultra-High speed InGaAsP/InP DFB lasers emitting at 1.3 μm wavelength”, IEEE Journal of Quantum Electronics, 1987, Volume QE-23, No. 6, pp. 1054-1058 and A. Valster, L. J. Meuleman, P. I. Kuindersma, T. V. Dongen, “Improved High-frequency Response of InGaAsP Double-channel Buried-heterostructure lasers”, Electronics Letters, 1986, Vol. 22, No. 1, pp. 17-18. In this structure, a pn-current block layer which may cause a device capacity is omitted to reduce a pn junction area for minimizing a capacity.
For a ridge waveguide type layer, there has been also disclosed, as means for reducing a device capacity, a structure in which an active layer area not contributing light emission is separated by a groove, by L. Bo, E. Vail, J. S. Osinski, B. Schmitt, “High-speed low-parasitic low-divergence 635 nm single mode lasers”, Electronics Letters, 1998, Vol. 34, No. 18, pp. 1750-1751. By the structure, a ridge waveguide type laser may have a reduced capacity to achieve high-speed response.
In the prior art described in these references, there is room for improvement in respect to the followings.
First, the technique in Japanese Patent Application No. 2000-138413, has a problem that when a diffraction grating is deep, a step tends to be formed, probably leading to deterioration in crystal quality. Furthermore, a separate step of Deep UV exposure results in increase in the number of production steps.
Second, the technique in Japanese Patent Application No. 11-354887 has a problem that the technique is effective to some extent to a step in a direction of a laser cavity, but is insufficiently effective for improving depth distribution in a direction perpendicular to an axial direction of the cavity.
Third, the methods disclosed by Kamite, et al., A. Valster, et al. and L. Bo, et al. are directed to reducing a device capacity for improving high-speed response properties of a device, but not to preventing deterioration in crystal quality due to drawing ends of a diffraction grating or deteriorated properties or reliability associated therewith.